The fastener is set to a panel member in a manner that the tubular portion is inserted into the hole of the panel, and a pulling stem or mandrel having an external threaded end is screwed into the nut portion through the tubular portion. By pulling the mandrel while pressing the flange portion against the panel, the tubular portion is outwardly radially expanded at the rear side of the panel to secure the fastener to the panel. The secured fastener is used as a blind nut for another screw member.
An automatic fastener setting tool is shown in the Japanese Patent Publication No. 53-4674 in which the tool comprises a spindle operatively connected to a motor through reduction gear means, a nut member operatively connected to the spindle through clutch means, a pulling rod threadably connected to the nut member extending forward to the tool with the end thereof fixedly securing the mandrel which extends outwardly from a nose piece for threadably supporting the fastener to set it to the panel, and a specific coil spring between the nut member and the pulling rod. In the operation of this tool, when the spindle is rotated in the non-inverting or normal direction, the nut member is normally rotated via the clutch means to reduce the diameter of the coil spring between the nut member and the pulling rod to increase the frictional engagement force of the nut member and the pulling rod for rotation of the pulling rod. Then, the mandrel secured to the pulling rod is rotated so that the end thereof begins to be screwed into the internally threaded portion of the fastener mounted on the panel, and, by consequent rotation, the flange portion of the fastener presses the nose piece rearward to bring the rear end of the nose piece into frictional engagement with the end of the pulling rod to prevent the pulling rod from rotating. Thus, where the pulling rod is retracted rearward by a predetermined length, a projection of the pulling rod is engaged with a projection of the tool body to fully prevent the rod from rotating. The unrotated pulling rod is further retracted rearward while the tubular portion is expanded to complete the setting. Upon completion, a greater axial force is applied to the pulling rod so that the clutch elements between the nut member and the spindle begin to slide relative to each other so as to stop rotation of the nut member. In order to remove the mandrel from the fastener, the spindle is reversably rotated to connect the clutch means and reversably rotate the nut member. Then, the diameter of the coil spring is increased to decrease the engagement force between the nut member and the pulling rod. However, since the two different projections are engaged with each other and the pulling rod is into frictional engagement with the rear end of the nose piece after the projections have been disconnected, the pulling rod is returned forward without rotation and is moved until a stopper at the rear end of the pulling rod abuts against the nut member. On the abutment, there is no connection of the projections and no engagement of the pulling rod and the rear end of the nose piece, so that the pulling rod is reversely rotated together with the nut member and the mandrel also is reversely rotated so as to be removed from the fastener.
In the known setting tool, frictional engagement or disengagement between the nut member and the pulling rod is due to reduction or increase of the spring diameter by utilizing the helical spring. However, attainment of effective functions is limited to only a case where a precise clearance is provided between the inner diameter of the spring and the outer diameters of the pulling rod and the nut member, and it is almost impossible to obtain such a precise clearance. Even if it is provided, abrasion of the engagement portion of the spring which is always frictionally moved is very significant and the frictional engagement force is reduced in a short period of time. It is also difficult to maintain the frictional engagement force between the pulling rod and the rear end of the nose piece for a long period, so that the reversely rotated pulling rod cannot be returned to the foremost position while the projections are engaged with each other. Thus, immediately after the beginning of the next operation, the pulling rod is suddenly retracted, so that the normal operation of the setting tool cannot be expected. Rotation of both the pulling rod and the nut member is due to the frictional force by the reduced coil spring. However, as the frictional force is smaller than the torque for screwing the mandrel into the fastener, the mandrel is retracted from an uncompletely screwed condition to provide insufficient setting.
Another tool known in the prior art has a mandrel and a pulling rod with a reversably threaded surface. When a pulling load is appied to the pulling rod, the mandrel is retracted but not rotated by the rod, while under no pulling load, the mandrel is rotated to return to the front position.
This tool is disadvantageous in that the rotating connection between the mandrel and the pulling rod is retained from the beginning of the retraction and therefore, the mandrel is rotated and retracted, so that the threaded portion of the fastener is apt to be damaged and the mandrel is apt to be broken by torsional stress. The externally threaded portion of the pulling rod is not always engaged with the internally threaded portion of the tool body and the engagement and disengagement are always repeated, so that the threaded portions become damaged. It is usual to press the tool against the fastener upon the reversed rotation for removal of the tool from the fastener. In this case, there is a disadvantage that the pulling rod strikes on the end of the threaded portion of the tool body which will thereby be abraded or damaged.